1. Field of the Invention
This invention relates to a laser engraving machine for engraving a workpiece surface, having a first laser beam source, a modulator in the beam path of the first laser beam source, an optical system following the modulator and spaced from the workpiece surface, the optical system and the workpiece being moved relative to each other, a control device driving the modulator with a first control signal so that its output radiation impinging on the workpiece surface is modulated according to the first control signal and machines the workpiece surface accordingly deeply, and a second laser beam source which is driven by the control device with a second control signal.
2. Description of the Related Art
Such laser engraving machines are known. The laser beam source characteristically consists of a CO.sub.2 laser which is driven by the control device with a control signal which depends on the desired machining profile of the workpiece surface. The workpiece surface is moved relative to the optical system. The workpiece is characteristically a cylinder made of rubber or plastic for example, which is clamped in a laser engraving machine where it is rotated and simultaneously moved translationally parallel to its rotation axis so that the laser beam focused on the workpiece surface by the optical system scans the workpiece surface and removes the workpiece surface more or less deeply in accordance with the control signal by corresponding beam intensity.
FIG. 1 shows a laser engraving machine which partly corresponds to the prior art.
Workpiece 2 is a cylinder made of rubber or having rubber surface 22. Workpiece 2 is rotated and moved translationally in the direction of the arrows.
Stationary laser engraving machine 4 contains a PC (personal computer) with interface IF as a control device and signal generator. Control device 6 delivers to first CO.sub.2 laser 8 control signal 1 which causes first laser 8 to apply output laser radiation to acoustooptic modulator 12, this output laser radiation of first laser 8 being linearly polarized and having an unvarying amplitude.
Via interface IF of control device 6 control signal S3 is applied to acoustooptic modulator 12, causing modulator 12 to modulate the laser beam from first laser 8 in accordance with the signal fluctuations of control signal S3 such that the output laser beam from acoustooptic modulator 12 has fluctuations of intensity corresponding to control signal S3.
The structure and use of acoustooptic modulator 12 are known in principle. It characteristically involves a crystal and a piezoelectric element so that when the piezoelectric element is driven acoustic waves are sent through the crystal which influence its optical properties. The laser beam passing through the modulator is diffracted in accordance with the frequency of the acoustic wave, i.e. the power or intensity of the output beam from modulator 12 is modulated, whereby working beam 1 emerging from the modulator is the beam of the first order of diffraction of the modulator.
The output laser beam from acoustooptic modulator 12 passes via optical system 18, where the radiation is focused, onto workpiece surface 22.
The power of the laser beam impinging on workpiece surface 22 thus varies in accordance with control signal S3, and since this laser beam scans workpiece surface 22 at constant speed, a profile corresponding to control signal S3 arises in workpiece surface 22. High beam power results in great machining depth, low beam power in small machining depth.
A laser engraving machine of the abovementioned kind, as is known from DE 42 12 390 A1, is constructed such that the two laser beams from the laser beam sources are directed before the workpiece via separate optical paths onto the workpiece surface so as to produce there a beam spot condensed by exact overlap of several beams, or else a multipart beam spot in which several partial beams are combined in a certain pattern in partial overlap or no overlap at all. U.S. Pat. No. 4,947,023 discloses a laser engraving machine wherein two laser beams are guided coaxially before the optical system in a partial area of their particular optical paths. This is intended to obtain a double engraving depth in the workpiece if both lasers are switched on during the work cycle.
DE 37 14 504 A1 discloses a laser engraving machine using two lasers with different wavelengths. The two laser beams of different wavelength are brought together on one machining spot.
It is known to modulate directly the operation of first laser 8 with the aid of control device 6 to obtain the desired machining profile on workpiece surface 22. However, a typical CO.sub.2 laser has a maximum modulation frequency in the kilohertz range, which prohibits fast machining of workpiece surfaces at least when the desired profile has very fine structures.
The use of the acoustooptic modulator permits faster and finer machining of the workpiece surface, because typical acoustooptic modulators have a maximum modulation frequency in the megahertz range.
However, the use of acoustooptic modulators is restricted because such a modulator can usually only modulate a laser beam with a maximum power of 100 watts. In many cases of application, for example when graduating press cylinders, it is imperative to obtain a certain profile depth. At a given upper power limit due to the modulator and at a given minimum profile depth at the bottom of the profile formed in workpiece surface 22 the machining speed is consequently restricted, since a certain minimum energy must be applied to the workpiece surface via the laser beam to attain the required profile depth in the workpiece surface. It follows that the laser beam having maximum power can only be moved across the workpiece surface relatively slowly.
The invention is based on the problem of providing a laser engraving machine of the abovementioned kind which can form fine contours, on the one hand, and obtain a certain minimum profile depth, on the other hand, while machining the workpiece surface fast.